In the past, many of the composite structures designed and fabricated for aerospace use have been composed of thin fabric laminate and sandwich skins, which require relatively light pressure on the composite material during the curing operation. Consequently, thin plastic film pressure bags can be used to apply pressure to these laminated composite structures during the curing operation. In such composite structures, woven fabric is commonly used to help maintain the fiber alignment and uniform wall thickness.
Following the development of fatigue resistant, unwoven, preimpregnated tapes of monofilament fiber, these tapes were used to fabricate heavy load-bearing structures such as the spar of a composite aerodynamic rotor blade assembly similar to that described in U.S. Patent No. 4,095,322 to Scarpatti et al., issued June 20, 1978. However, light pressure devices such as the thin film plastic pressure bags earlier described were found to be unsuitable for these dense, thickwalled composite structures. As a result of a large bulk factor (the ratio of cured/uncured wall thickness), fiber drift, uneven resin flow, and entrapped air in the composite, when these heavy laminated structures were formed using thin film pressure bags, the cured product invariably exhibited many irregularities on the inside wall surface and a lack of consistent wall thickness. Further, the rough inner wall surface greatly increased the risk of failure of the fine film pressure bag by puncturing, which also resulted in a defective product.
For these reasons, until the present invention, rigid caul plates and heavy elastomeric pressure bags of silicone rubber, having a wall thickness in the range of 1/16 to 1/2 inch, have been used to form these dense, thickwalled, heavy load-bearing, composite structures.
The initial fabrication cost of the silicone rubber pressure bags is very high, in comparison to thin film plastic bags, and the characteristics of silicone rubber are such that the entire surface of the silicone rubber pressure bag must be covered with Teflon tape, which acts as a release medium. Also, damage to the silicone rubber pressure bag frequently occurs during the removal of this bag from the cured composite structure, necessitating costly bag repairs and retesting for leaks.